.\" $NetBSD: carp.4,v 1.8 2020/10/12 15:18:48 roy Exp $ .\" $OpenBSD: carp.4,v 1.19 2005/08/09 09:52:12 jmc Exp $ .\" .\" Copyright (c) 2003, Ryan McBride. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd October 12, 2020 .Dt CARP 4 .Os .Sh NAME .Nm carp .Nd Common Address Redundancy Protocol .Sh SYNOPSIS .Cd pseudo-device carp .Sh DESCRIPTION The .Nm interface is a pseudo-device which implements and controls the CARP protocol. .Nm allows multiple hosts on the same local network to share a set of IP addresses. Its primary purpose is to ensure that these addresses are always available, but in some configurations .Nm can also provide load balancing functionality. .Pp A .Nm interface can be created at runtime using the .Ic ifconfig carp Ns Ar N Ic create command. .Pp To use .Nm , the administrator needs to configure at minimum a common virtual host ID and virtual host IP address on each machine which is to take part in the virtual group. Additional parameters can also be set on a per-interface basis: .Cm advbase and .Cm advskew , which are used to control how frequently the host sends advertisements when it is the master for a virtual host, and .Cm pass which is used to authenticate carp advertisements. Finally .Cm carpdev is used to specify which interface the .Nm device attaches to. If unspecified, the kernel attempts to set carpdev by looking for another interface with the same subnet. These configurations can be done using .Xr ifconfig 8 , or through the .Dv SIOCSVH ioctl. .Pp Additionally, there are a number of global parameters which can be set using .Xr sysctl 8 : .Bl -tag -width xxxxxxxxxxxxxxxxxxxxxxxxxx .It net.inet.carp.allow Accept incoming .Nm packets. Enabled by default. .It net.inet.carp.preempt Allow virtual hosts to preempt each other. It is also used to failover .Nm interfaces as a group. When the option is enabled and one of the .Nm enabled physical interfaces goes down, advskew is changed to 240 on all .Nm interfaces. See also the first example. Disabled by default. .It net.inet.carp.log Log bad .Nm packets. Disabled by default. .It net.inet.carp.arpbalance Balance local traffic using ARP. Disabled by default. .El .Sh EXAMPLES For firewalls and routers with multiple interfaces, it is desirable to failover all of the .Nm interfaces together, when one of the physical interfaces goes down. This is achieved by the preempt option. Enable it on both host A and B: .Pp .Dl # sysctl -w net.inet.carp.preempt=1 .Pp Assume that host A is the preferred master and 192.168.1.x/24 is configured on one physical interface and 192.168.2.y/24 on another. This is the setup for host A: .Bd -literal -offset indent # ifconfig carp0 create # ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1 \e netmask 255.255.255.0 # ifconfig carp1 create # ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1 \e netmask 255.255.255.0 .Ed .Pp The setup for host B is identical, but it has a higher advskew: .Bd -literal -offset indent # ifconfig carp0 create # ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat \e 192.168.1.1 netmask 255.255.255.0 # ifconfig carp1 create # ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \e 192.168.2.1 netmask 255.255.255.0 .Ed .Pp Because of the preempt option, when one of the physical interfaces of host A fails, advskew is adjusted to 240 on all its .Nm interfaces. This will cause host B to preempt on both interfaces instead of just the failed one. .Pp In order to set up an ARP balanced virtual host, it is necessary to configure one virtual host for each physical host which would respond to ARP requests and thus handle the traffic. In the following example, two virtual hosts are configured on two hosts to provide balancing and failover for the IP address 192.168.1.10. .Pp First the .Nm interfaces on Host A are configured. The .Cm advskew of 100 on the second virtual host means that its advertisements will be sent out slightly less frequently. .Bd -literal -offset indent # ifconfig carp0 create # ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10 \e netmask 255.255.255.0 # ifconfig carp1 create # ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat \e 192.168.1.10 netmask 255.255.255.0 .Ed .Pp The configuration for host B is identical, except the skew is on virtual host 1 rather than virtual host 2. .Bd -literal -offset indent # ifconfig carp0 create # ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat \e 192.168.1.10 netmask 255.255.255.0 # ifconfig carp1 create # ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10 \e netmask 255.255.255.0 .Ed .Pp Finally, the ARP balancing feature must be enabled on both hosts: .Pp .Dl # sysctl -w net.inet.carp.arpbalance=1 .Pp When the hosts receive an ARP request for 192.168.1.10, the source IP address of the request is used to compute which virtual host should answer the request. The host which is master of the selected virtual host will reply to the request, the other(s) will ignore it. .Pp This way, locally connected systems will receive different ARP replies and subsequent IP traffic will be balanced among the hosts. If one of the hosts fails, the other will take over the virtual MAC address, and begin answering ARP requests on its behalf. .Pp Note: ARP balancing only works on the local network segment. It cannot balance traffic that crosses a router, because the router itself will always be balanced to the same virtual host. .Sh SEE ALSO .Xr netstat 1 , .Xr sysctl 3 , .Xr arp 4 , .Xr arp 8 , .Xr ifconfig 8 , .Xr sysctl 8 .Sh HISTORY The .Nm device first appeared in .Ox 3.5 .